Proposed genetic model for the precipitation of uranium in Siwaliks of Taunsa area, D.G. Khan, Pakistan
Abstract
The fluvial rocks, mostly Siwaliks, comprise molasse sediments which are deposited during middle Miocene to Pleistocene. The middle Siwaliks of the area is the host rocks for uranium exploration in D.G. Khan Division. Two distinct types of ore deposits have been discovered so far in the Siwalik rocks of D. G. Khan, i.e. paleochannel type & chemical ore deposit.
The eastern limb of the Girdu anticline has a habit of rendering paleochannel type of ore deposits whereas the eastern limb of the Zinda Pir anticline is holding a unique type ofuranium accumulation Which may be termed as chemical ore body. This uranium accumulation has no corresponding radioactive signatures and the mineral accumulation appears to be quite young. The genetic model for this accumulation is interpreted as the secondary uranium which was formed along with fluvial sediments and got enriched due to subsurface water movement. The orogenic movements caused uplifting of these fluvial rocks due to which erosional surfaces developed. As a result, uranium got liberated through dissolution by water and was mobilized to the paleowater tables. During episodic uplifts the process is repeated manifold and the last phase of uplifting has established the present day water table. Due to Eh-pH condition of the subsurface water, the remobilized uranium in the form of uranyl complexes reached the redox boundary (-ve Eh condition) where it changed its valency from U+6 to U+4 and got stabilized. Thus the stabilized U+4 precipitated at the redox interface due to change in Eh-pH conditions and formed chemical ore body. The Lal-Ashab uranium deposit of Taunsa is a similar ore accumulation which may be called as a hanging ore body existing at a depth of 45-50 m, 20 m below the present- day water table in a tabular shape. The mineral could not exactly be identified through ore microscopy and XRD, however, due to its young age the uranyl oxidized variety exists between U3O8 & UO2. Some of the gamma logs show three levels of radioactivity which either show age difference or repeated mobilization scenario.
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